Recently I did a quick island hop and went over to Lombok, Bali's neighbouring island. Due to the small distances between the two and the cost of ferries, this is much easier than you might think. The ferry takes between 4-6 hours depending on which ship you happen to take.
Firstly, I should note that I took the ferry, which is an interesting voyage to say the least. It is around $3.50 per person one way - very reasonable. If you plan to go by motorbike